In order to obtain good selling between a receptacle and its lid, it is necessary for the gasket to be somewhat flexible, and naturally it is also necessary for it to withstand chemically the substance that is to be contained in the receptacle. In order to obtain flexibility, it is common practice to use rubber. There are numerous different natural and synthetic rubber formulas having different hardnesses, and different qualities of chemical resistance. Unfortunately, there are numerous substances, to be found in particular perfumery and in medicines, which attack or dissolve all types of rubber: the gasket ceases to provide sealing and the substance contained in the receptacle may be unacceptably polluted or degraded.
There are numerous other flexible materials which stand up well to coming into contact with the substances used in the perfumery, pharmacy, and other chemical industries, for example there are the synthetic plastic materials known under the trademarks NYLON and TEFLON (polytetrafluoroethylene or PTFE). However, these substances are not flexible enough to obtain the desired degree of sealing. Attempts have therefore been made to combine these substances, e.g. by coating a rubber gasket with a thin layer of TEFLON.
On these lines, German patent application DE 2 647 524 filed in 1976 by the Japanese company Daikin Kogyo Co. Ltd. suggests rolling a film of TEFLON onto a sheet of rubber and then vulcanizing the assembly. As emphasized in this prior publication, the method nevertheless suffers from a practical difficulty: normal TEFLON films do not adhere to rubber. In order to obtain the desired adherence, experience shows that it is necessary to use TEFLON which has been made porous by some appropriate method: sintering or extruding PTFE powder in an appropriate solvent; weaving or agglomerating fibers of PTFE; or indeed obtaining separation in the form of a PTFE foam. Unfortunately, gaskets made in this way remain relatively vulnerable to chemical attack since the aggressive substances can filter through the pores in the TEFLON film and reach the rubber. That is why the above-mentioned prior application recommends interposing a layer of glue between the rubber and the porous TEFLON film. However, the application of an intermediate glue constitutes an additional operation which naturally gives rise to additional cost.
The object of the present invention is to avoid such extra cost and thus to seek a method for causing a film of TEFLON to adhere directly on rubber, while ensuring that the TEFLON film is of a structure suitable for providing protection from chemical attack by aggressive substances on a long term basis.